Coupling efficiency of monolithic,
waveguide-integrated Si photodetectors
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Ahn, Donghwan, Ching-Yin Hong, Lionel C. Kimerling, and Jurgen
Michel. “Coupling efficiency of monolithic, waveguide-integrated Si
photodetectors.” Applied Physics Letters 94, no. 8 (2009): 081108. ©
2009 American Institute of Physics.
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Coupling efficiency of monolithic, waveguide-integrated Si photodetectors
Donghwan Ahn, Ching-yin Hong, Lionel C. Kimerling, and Jurgen Michel
Citation: Appl. Phys. Lett. 94, 081108 (2009); doi: 10.1063/1.3089359 View online: http://dx.doi.org/10.1063/1.3089359
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Coupling efficiency of monolithic, waveguide-integrated Si photodetectors
Donghwan Ahn,a兲 Ching-yin Hong, Lionel C. Kimerling, and Jurgen Michelb兲
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
共Received 16 October 2008; accepted 26 January 2009; published online 24 February 2009兲 A waveguide-integrated photodetector provides a small-footprint, low-capacitance design that overcomes the bandwidth-efficiency trade-off problem of free space optics. High performance silicon devices are critical to the emergence of electronic-photonic integrated circuits on the complementary metal oxide semiconductor platform. We have fabricated vertical p-i-n silicon photodetectors that are monolithically integrated with compact silicon oxynitride channel waveguides. We report over 90% coupling efficiency of 830 nm light from the silicon oxynitride 共SiOxNy兲 channel waveguide to the silicon photodetector. We analyze the dependence of coupling on waveguide index by comparing coupling from low index-contrast waveguides and high index-contrast waveguides. © 2009 American Institute of Physics.关DOI:10.1063/1.3089359兴
Monolithic integration of complementary metal oxide semiconductor compatible waveguides 共WGs兲 and photode-tectors共PDs兲 is one of the most important technologies in the emerging Si-based integrated photonics.1 A bottom-WG-coupled structure has been commonly adopted in most III-V compound semiconductor-based WG-integrated PDs2,3 and more recent Ge PDs integrated with Si WGs,4,5 as the WG and PD materials are sequentially grown by a heteroepitaxial process. On the other hand, a top-WG-coupled structure may fit better into the intrachip optical interconnection architectures6because it retains the general Si chip structure, where active devices are fabricated on the substrate and the passive interconnections are built in the upper levels.
In addition to a recent report of top-WG-integrated Ge PD,7 there have been a few previous reports of Si PDs8–11 integrated with upper dielectric WGs. However, most of them exhibited slow evanescent wave coupling rates and amorphous10 or polycrystalline11 silicon detectors, which also suffered from low quantum efficiency and high bias voltage requirements. In this paper, we present crystalline
p-i-n silicon PDs integrated with compact SiON channel
WGs, which demonstrated high coupling efficiency and high responsivity with relatively small device size due to high evanescent coupling rates.
A top-WG-integrated structure provides great flexibility in the materials choices and device designs. Therefore, it is critical to understand the trends in coupling behavior and device performance dependency on design parameters, which were not addressed in previous studies8–11as they re-ported on specific material and design choices. In our current work, we report the dependence of coupling efficiency on WG design and the resulting impact on PD performance. The results can provide design guidance for a broader range of evanescent coupling structures with various materials in sili-con microphotonics.7
For device fabrication, Si p+ wafers共0.01–0.02 ⍀ cm兲 with a 5 m thick epitaxial Si layer 共16–24 ⍀ cm兲 were
used. After the epitaxial layer was etched down to 0.5 m thickness, phosphorus was implanted with a dose of 3 ⫻1015/cm2 at 100 keV, followed by an activation annealing
at 1000 ° C for 30 s. Silicon vertical p-i-n diodes were pat-terned and a 2.2 m depth was plasma-etched. This was done in order to ensure optical isolation of the WGs from the Si substrate. Then, 3.5 m SiO2 was deposited over the Si
PD mesas and then planarized and polished down by chemi-cal mechanichemi-cal polishing, leaving a thin oxide above the Si mesas. Patterning and opening of oxide windows followed in order to expose the top surface of the silicon PDs. Next, the single-mode WGs were formed by plasma enhanced chemi-cal vapor deposition of SiOxNyfollowed by etching to define the WGs. Both relatively low index-contrast WGs 共1.2 ⫻1.2 m2 with n
core= 1.52兲 and higher index-contrast,
smaller WGs 关1.2 m共width,W兲⫻0.4 m共height,H兲 with
ncore= 1.58 and 0.9 m共W兲⫻0.3 m共H兲 with ncore= 1.67兴
were prepared. The dimensions of the WGs were chosen for single-mode operation and to stay within the resolution limit of the available lithography tool. The index contrast is de-fined by ⌬n=ncore− nclad. A 2.2 m thick SiO2 cladding layer 共nclad= 1.45兲 was deposited, followed by opening
con-tact holes at least 3 m away from the WG in order to prevent the metal from interacting with the optical mode. After depositing and patterning Al contact pads, the wafers were annealed in N2/H2forming gas. For testing, while cou-pling 830 nm transverse-electric light to the cleaved WG input facets, we simultaneously measured the photocurrent
Iph at ⫺2V reverse bias and the remaining optical power
from the output facet of the through WGs Pout. The device
structures are shown in Fig.1.
Figure 2共a兲 shows the transmitted optical power
Pout共WG-PD兲, normalized to the power at the reference WG
Pout共ref-WG兲, as a function of detection length L. As the data
show the dependence of exponential decay on the coupling length L, we can fit the data with the following equation:
Pout共WG-PD兲共L兲 Pout共ref-WG兲 =
Pininexp共−␣couplingL兲out10−␣WGl⬘/10
Pin10−␣WGl/10
⬇2exp共−␣
couplingL兲, 共1兲
where Pin is the optical power at the point of entering the
a兲Present address: Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, Texas.
b兲Author to whom correspondence should be addressed. Electronic mail: jmichel@mit.edu.
APPLIED PHYSICS LETTERS 94, 081108共2009兲
PD. Because the optical mode in the WG on top of the PD is different from the mode in the bus WG, we introduced mode coupling efficiencies inandout that describe the coupling
from the bus WG mode to the mode in the WG on top of the PD and vice versa. In approximation, it was assumed thatin and out are equal and that the term 10−␣WG共l⬘−l兲/10 is
negli-gible, as the propagation loss in the WG␣WG is small.
Fitting the measurement data to Eq. 共1兲 suggests that incoming photons from the input bus WG are coupled to a mode in the WG on the PD within⬃86.3% mode-matching efficiency and the power in the WG on the PD decreases at a rate of 1/␣coupling= 52 m due to the evanescent wave
pling to the Si PD. We can also conclude that the total cou-pling efficiency of photons from the input bus WG to silicon
PD is well above 90% because nearly all of the photons that are coupled to the WG mode on the PD共⬃Pin⫻in兲 will be
coupled to the Si PD via evanescent wave coupling in a sufficiently long PD 关e.g., L⬎3⫻共1/␣coupling兲兴, and some
portion of the remaining 14% photons共⬃Pin⫻共1-in兲兲 also
can contribute to the photocurrent by being directly coupled to the Si detector.
The measured quantum efficiency as a function of detec-tor length L is shown in Fig. 2共b兲 and can be described as follows: QE共L兲 = h qPin⫻ Iph共L兲 = h qPout共ref-WG兲10−␣WGl/10⫻ Iph共L兲 ⬇ A + C关1 − exp共−␣detL兲兴. 共2兲
The saturated quantum efficiency, represented by the term 共A+C兲 in Fig.2共b兲, was about 46%, slightly higher than the efficiency of Si PD when measured with surface-normal il-lumination共41%兲.
The results of coupling from high index-contrast WGs, shown in Fig.3, reveal two significant differences compared to low index-contrast WGs. First, the evanescent coupling to the detector occurs much faster than for low index-contrast WGs. The greater index-contrast between core and cladding of a WG 共ncore− nclad兲 leads to smaller refractive index
dif-ference between the PD and WG core 共nPD− ncore兲. In
addi-tion to the smaller index difference between WG and detec-tor, the geometrical factors of these WGs, such as smaller WG dimensions to maintain the single-mode condition and reduced height, also affect the impedance of the WG layer
FIG. 1. 共a兲 Schematic structure of a top-WG-coupled Si p-i-n PD device. 共b兲 Cross-section of the WG-coupled PD device. Some notations used in the equations are shown.
FIG. 2. Measured and fitted data of 共a兲 normalized optical power output through the WGs coupled with PDs and共b兲 PD quantum efficiency vs de-tector coupling length. The sample is coupled with a relatively low index-contrast WG共1.2⫻1.2 m2with n
core= 1.52兲.
FIG. 3. Measured and fitted data of 共a兲 normalized optical power output through the WGs coupled with PDs and共b兲 PD quantum efficiency vs de-tector coupling length. The samples are coupled with relatively high index-contrast WGs 共1.2⫻0.4 m2 with n
core= 1.58 and 0.9⫻0.3 m2 with ncore= 1.67兲.
through an effective index change 共i.e., Z⬀1/
冑
ncore2 − neff2 兲 and thus lower the impedance barrier between WG and PD. As a result, a leaky mode has more evanescent field penetrat-ing into silicon and the evanescent couplpenetrat-ing rate ␣coupling is higher, as demonstrated by much shorter 1/␣coupling andquick rise in the quantum efficiency within short coupling lengths 共e.g., ⬍40 m兲 as shown in Fig.3.
The second significant difference between low versus high index-contrast WGs is the observed direct coupling from the bus WG to modes inside the Si PD. Figure 4共a兲 shows the simulation results for such a scenario. The simu-lation clearly shows that the light is coupled to modes in the Si PD. Simulations for the low index-contrast case 关Fig.
4共b兲兴 show that the light remains in the WG and generally
does not couple directly into the Si PD. The direct coupling into the Si PD explains the relatively low in Fig. 3共a兲, indicating that only a small fraction of the light from the bus WG remains in the WG on top of the detector.
The lower quantum efficiencies for the low-index-contrast WGs are mainly due to the fact that during evanes-cent coupling, photons enter the PD nearly perpendicular to the WG-detector interface. Due to the long absorption length of 13 m for 830 nm light in Si, only 41% of the photons can be absorbed in the top 7 m thick layer共given the ver-tical p-i-n geometry of our silicon PD, only the photons ab-sorbed within a certain depth of the PD—top 7 m in our devices—can contribute to the photocurrent兲. Therefore, the quantum efficiency of the PD coupled with low
index-contrast WG is close to the efficiency of the PD when mea-sured with surface-normal illumination. In contrast, for the high index-contrast WG cases, the modes, excited due to direct coupling to the Si PD, travel in the horizontal direction and therefore are not limited by absorption length of Si. As the absorption length limitation is lifted, the quantum effi-ciency increases significantly.
In summary, we have demonstrated WG-integrated sili-con PDs that employ a top-to-bottom evanescent-wave cou-pling structure, achieving over 90% coucou-pling efficiency. PD devices integrated with single-mode, low index-contrast WGs exhibit relatively slow evanescent coupling rates and a slightly higher quantum efficiency than that of surface-illuminated device. In the case of single-mode, high index-contrast WGs, coupling to PDs occurred mainly by mode matching of the input WG mode and modes in the Si PD, leading to an enhanced 80% quantum efficiency. The ob-served trend can generally be extended to a wider variety of top WG-coupled vertical p-i-n PDs, as was recently demon-strated by an integrated Ge PD coupled with silicon nitride WGs.7
1L. C. Kimerling, L. Dal Negro, S. Saini, Y. Yi, D. Ahn, S. Akiyama, D. Cannon, J. Liu, J. G. Sandland, D. Sparacin, J. Michel, K. Wada, and M. R. Watts, in Silicon Photonics: Topics in Applied Physics, edited by L. Pavesi and D. J. Lockwood共Springer, Berlin, 2004兲, Vol. 94, p. 89. 2R. J. Deri, W. Doldissen, R. J. Hawkins, R. Bhat, J. B. D. Soole, L. M.
Schiavone, M. Seto, N. Andreadakis, Y. Silberberg, and M. A. Koza,
Appl. Phys. Lett. 58, 2749共1991兲.
3Y.-S. Wu, J.-W. Shi, J.-Y. Wu, F.-H. Huang, Y.-J. Chan, Y.-L. Huang, and R. Xuan,IEEE Photonics Technol. Lett. 17, 878共2005兲.
4J. F. Liu, D. Ahn, C. Y. Hong, D. Pan, S. Jongthammanurak, M. Beals, L. C. Kimerling, J. Michel, A. T. Pomerene, C. Hill, M. Jaso, K. Y. Tu, Y. K. Chen, S. Patel, M. Rasras, A. White, and D. M. Gill, Proceedings of the Optics Valley of China International Symposium on Optoelectronics, Wu-han共IEEE, New York, 2006兲, p. 1.
5T. Yin, R. Cohen, M. M. Morse, G. Sarid, Y. Chetrit, D. Rubin, and M. J. Paniccia,Opt. Express 15, 13965共2007兲.
6A. V. Mule, E. N. Glytsis, T. K. Gaylord, and J. D. Meindl,IEEE Trans.
Very Large Scale Integr.共VLSI兲 Syst. 10, 582共2002兲.
7D. Ahn, C.-Y. Hong, J. Liu, W. Giziewicz, M. Beals, L. C. Kimerling, and J. Michel,Opt. Express 15, 3916共2007兲.
8S. Wunderlich, J. P. Schmidt, and J. Muller, Appl. Opt. 31, 4186共1992兲. 9U. Hilleringmann and K. Goser, IEEE Trans. Electron Devices 42, 841
共1995兲.
10K. Kapser and P. P. Deimel, J. Appl. Phys. 71, 3614共1992兲.
11G. Yuan, R. Pownall, P. Nikkel, C. Thangaraj, T. W. Chen, and K. L. Lear,
IEEE Photonics Technol. Lett. 18, 1657共2006兲.
FIG. 4. 共Color online兲 Cross-section view of the mode profile immediately 共at 0.5 m兲 after coupling region to the PD begins. 共a兲 In the coupling structure with smaller high index-contrast WG with flat rectangular shape 共0.9⫻0.3 m2,⌬n=0.22兲 共b兲 In the coupling structure with lower index-contrast WG with larger square dimensions共1.2⫻1.2 m2,⌬n=0.07兲.
